Cholesterol is known to be an important contributor to coronary heart disease, a leading cause of death in the United States. As a result, many consumers are aware of the benefits of maintaining a diet that is low in saturated fat and cholesterol. According to one survey, 48% of US adults try to limit the amount of dietary cholesterol. Council for Agricultural Science and Technology Report #107, November 1985. Although dietary cholesterol intake does not determine blood cholesterol levels in 70-80% of U.S. adults, a significant market for reduced cholesterol products remains for 20-30% of U.S. adults. Sperber, Food Processing, November 1989, page 155.
As the link between high serum cholesterol levels and heart disease has become increasingly apparent, cholesterol-free and cholesterol-reduced food products have become more attractive to consumers, and food products that have no or reduced cholesterol are gaining popularity as well as an increasing share of the market. Consequently, removal or reduction of cholesterol in high cholesterol foods has the potential to substantially increase marketability and value. This is particularly true for such high cholesterol foods as marine fish oils (500-800 mg cholesterol/100 g), butter (250-300 mg cholesterol/100 g), beef tallow (about 110 mg cholesterol/100 g), lard (about 100 mg cholesterol/100 g) and egg yolks (about 5.2 g cholesterol/100 g (dry weight)).
Many fish oils contain essential omega-3 fatty acids such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), which are gaining importance as "nutraceuticals," i.e., food substances which, when ingested, are beneficial in treatment or prevention of disease. The benefits of increasing dietary DHA and EPA include reduction of blood pressure and plasma triacylglycerols, and alleviation of autoimmune diseases such as arthritis. DHA is an important structural component in the membranes of the eye and brain, and is very important in the development of infants. McNeill et al., J. Am. Oil Chem. Soc. 73: 245-51 (1996). Unfortunately, many fish oils display very high levels of cholesterol (see above), such that in order to obtain the benefits of DHA, EPA and other omega-3 fatty acids in fish oils, one must also ingest large quantities of cholesterol. Thus, removal of cholesterol from fish oils, without adversely affecting the omega-3 fatty acids and other beneficial constituents, would enhance the nutraceutical character of fish oil and would be particularly valuable from the perspective of human health.
The requirements for awarding a product a reduced-cholesterol or cholesterol-free label are stringent. From the labeling guidelines given below, the importance of high efficiencies in cholesterol removal is clear:
Cholesterol Free: &lt;2 mg cholesterol & 2 g or less saturated fat per serving PA1 Low Cholesterol: 20 mg or less cholesterol & 2 g or less saturated fat per serving PA1 Reduced Cholesterol: &gt;25% less cholesterol & 2 g or less saturated fat per serving than a comparison food
The Chemist, Vol. 71, No. 5, July, 1994 (newsletter published by The American Institute of Chemists, Inc.).
The removal or reduction of cholesterol is not a trivial matter. Several different techniques to accomplish this task have been developed, each with varying levels of success.
One such method, disclosed in Marschner et al., U.S. Pat. No. 4,804,555, involves steam distillation. In this process the oil or fat is heated to 500.degree. F. under high vacuum. Then the oil is made into a thin film and brought into contact with 1 to 15% steam under counter current conditions. This results in a 65% or more reduction of cholesterol. The high temperatures, however, can degrade the product by isomerizing cis double bonds present in triacylglycerols into trans double bonds. The high temperatures also can lead to polymerization and oxidation of various constituents, including cholesterol not removed in the processing steps. Oxidized cholesterol, even more than unoxidized cholesterol, is considered particularly detrimental to human health. Marine fish oils are especially vulnerable to these various forms of product degradation induced by high temperatures. The steam distillation process is energy intensive and engenders significant operating costs. Furthermore, since appreciable amounts of cholesterol remain after distillation, the resulting product may not be entitled to a cholesterol-free label.
Another method of cholesterol removal involves extraction with supercritical CO.sub.2. Sperber, "New Technologies for Cholesterol Reduction," Food Processing, November 1989, pages 155-60. Here, the CO.sub.2 is converted into a supercritical fluid using high pressures (4000-5000 psi). The supercritical CO.sub.2 has unique solvating characteristics and is used to selectively remove the cholesterol from dairy and animal products. The efficiency of such an extraction can be as high as 98%. However, installation of the necessary equipment requires a substantial capital investment and the operating costs are also substantial. See also McLachlan et al., U.S. Pat. No. 5,024,846 for another method of separating cholesterol from lipids using supercritical CO.sub.2.
Another method, disclosed in Wrezel, U.S. Pat. No. 5,128,162, involves chemical reaction and extraction of cholesterol from oils. In a first step, the cholesterol containing oil is reacted with a polybasic acylating agent. This converts the cholesterol into an acidic monoester of the corresponding polybasic acid. In a second step, this acidic cholesterol derivative is extracted with a base and separated by centrifugation. The chemical requirements, relatively complex procedure, and generation of waste can add significant costs to the process.
Modified cyclodextrin polymers and hydrocolloid extractants such as galactomannoses have also been used successfully to extract cholesterol. Other methods include use of calcium or magnesium bromide salts to precipitate cholesterol, as well as extraction using modified vegetable oils. Trends in Food Science and Technology 7: 30 (1996).